1. Field of the Invention
The invention relates to the catalytic purific action of engine exhaust streams to remove nitric oxide therefrom. More particularly, the invention relates to such a system wherein the nitric oxide content is reduced through reaction with ammonia.
2. Prior Art
It is well known to react a combusted gas stream from a hydrocarbon burning engine, which stream contains oxygen and nitric oxide among other components over a catalyst bed with ammonia, whereby the ammonia and nitric oxide react to reduce or substantially eliminate the nitric oxide. However, a problem has existed with such systems in that the ammonia to be effective must be in a high enough amount so that it serves to reduce substantially all of the nitric oxide to nitrogen. When this much ammonia is utilized, a problem is introduced in that excess ammonia escapes along with the exhaust stream from the catalytic reactor. At low loads, the mass flow of nitric oxide drops substantially. Consequently, not enough nitric oxide is supplied to react with ammonia absorbed on the catalyst surface at a sufficient rate to prevent the ammonia from being described and exhausted.